This invention relates generally to processing equipment used in the manufacturing of high density semi-conductor wafers. More particularly, it relates to an improved apparatus and a method for extracting of silicon dioxide from an etching bath utilizing silicon-nitride-etching.
As is generally known in the art of manufacturing of high density semiconductor wafers, various layers of silicon materials are typically formed on a substrate. Thereafter, it is also known that a silicon-nitride-etching with hot phosphoric acid process is used so as to remove unwanted silicon nitride from the surfaces of the semiconductor wafers. The phosphoric acid is used to strip the silicon nitride from the semiconductor wafers during the various processing steps.
The preferred chemical process for forming silicon dioxide and ammonia is obtained by reacting silicon nitride with water in the presence of phosphoric acid and is given by the following reaction: EQU Si.sub.3 N.sub.4 +6H.sub.2 O.fwdarw.3SiO.sub.2 +4NH.sub.3 H.sub.3 PO.sub.4
The phosphoric acid is utilized to function as a catalyst rather than serving as an etchant. A prior art silicon nitride etch bath system 10 is shown in FIG. 1 and is labeled as "Prior Art." The silicon nitride etch bath system 10 includes an etching bath 12, a pump 14, a filter 16, and a heater 18 all connected together in series to form a recirculation loop. The etching bath 12 contains a solution of phosphoric acid (H.sub.3 PO.sub.4) in which the semiconductor wafers (not shown) with silicon-nitride films are inserted for removing of the silicon-nitride. The phosphoric acid flowing into the outer weir 13 adjacent the sides of the bath 12 is sent to the pump 14. The pump is used to recirculate the phosphoric acid via the filter 16 and the heater 18 back to the bath. The temperature of the bath 12 is controlled by the heater 18 and by adding water thereto.
The problem arises as more and more semiconductor wafers are etched in the same solution of phosphoric acid so that the silicon dioxide (SiO.sub.2) reaction byproduct becomes more concentrated. The solubility limits for silicon (Si) in the form of silicon dioxide in the phosphoric acid (H.sub.3 PO.sub.4) at the temperature of 150.degree. C. are about 100 parts-per-million (ppm). Above this limitation, the silicon dioxide will come out of the solution as a solid precipitate or a source of particles. The solid precipitation of silicon dioxide will form as suspensions in solution which may clog the filter 16 and cause contamination on the wafers. As a result, the processing equipment may become inoperable and will be required to be serviced which increases the manufacturing costs as well as involving time-consuming operations. In addition, the chemical in the etching bath must be changed or replaced quite frequently when the solution therein becomes saturated, thereby also increasing production expense.
Further, the amount of silicon dioxide in the solution also modulates the etch-rate of silicon dioxide on the wafers. In practice, it is typically desirable to "season" the etching bath 12 of the phosphoric acid in order to slow down the etch rate of silicon dioxide, but not enough so as to saturate the solution and thus cause the precipitation of particles. Consequently, the etchrate will be varying which produces inconsistent etch results.
In a technical paper authored by K. Sato et al. of the Tohoku University, and entitled "Mechanistic Study of Silicon-Nitride Etching with Hot Phosphoric Acid," there is reported the results of an investigation conducted on silicon-nitride etching and the effects of reaction products to etching rate. It was determined that reaction products precipitated as particles when the concentration was more than 110 ppm of silicon at the temperature of 160.degree.0 C. In FIG. 6 of this paper, there is shown a graph of the solubility of the reaction product as a function of temperature. In particular, it can be seen from the graph that the concentration of silicon in the filtered phosphoric acid was reduced to approximately 20 ppm at the temperature of 50.degree. C.
The inventors of the present invention have utilized this general principle as taught by this aforementioned technical paper that the solubility of silicon in phosphoric acid is significantly decreased when it is cooled in order to realize the instant silicon nitride etch bath system. The silicon nitride etch bath system of the present invention represents a significant improvement over the prior art system of FIG. 1.